Aromatic polyesters, such as “polyarylates” and certain liquid crystalline polymers (LCPs) are important items of commerce. They are typically made by one of two processes. In one of the processes the aromatic carboxyl groups are converted to aryl esters (e.g. esters of phenol), and then reacted with the aromatic hydroxy compounds to form the polyesters and byproduct aromatic hydroxyl compound (e.g. phenol) which is removed by volatilization. More commonly used is the route that involves reaction of the aromatic carboxyl groups with aliphatic esters (e.g. acetates) of the aromatic hydroxyl groups with subsequent removal of the byproduct aliphatic acid (e.g. acetic acid).
Typically the aliphatic carboxylic (di)ester starting material is prepared by reaction of an aliphatic carboxylic anhydride (e.g. acetic anhydride) with the hydroxyl groups, and then that ester is reacted with the aromatic carboxyl groups to start the polymerization process. The equations for such reactions are given below:(R1CO)2O+HO—Ar1—OH→R1CO2H+R1CO—O—Ar1—O—OCR1  (1)R1CO—O—Ar1—O—OCR1+HO2C—Ar2—CO2H→R1CO—O—Ar1—O—OC—Ar2—CO2H+R1CO2H  (2)If an aromatic hydroxyacid is present it reacts with the anhydride as follows:HO—Ar3—CO2H+(R1CO)2O→R1CO—O—Ar3—CO2H+R1CO2H  (3)
In these equations R1 is alkyl, substituted alkyl, cycloalkyl, or substituted cycloalkyl, and Ar1, Ar2 and Ar3 are each independently arylene or substituted arylene. The aliphatic ester groups and aromatic carboxyl groups in the product of Equation (3) may then react analogously to those in reaction (2). Reaction (2), which is the condensation reaction, then repeats many times to form the polyester. Notice that in all of the steps the aliphatic carboxylic acid (ACA), R1CO2H, is a byproduct.
In the process using the aliphatic carboxylic (di)ester the aliphatic carboxylate esters may be formed beforehand and isolated, and then reacted with the aromatic carboxylic acid groups in a “separate” reaction, or they may be formed “in situ” in the presence of the aromatic carboxyl compounds. During the polymerization process, as the condensation of the monomers proceeds to form higher and higher molecular weight oligomers and then polymers, the reaction temperature is typically raised and the byproduct ACA is distilled off. Towards the end of the polymerization process vacuum may be used to remove the byproduct acid, and/or the lower molecular weight polymer or oligomer may be solidified and “solid state polymerized”. These various combinations of steps typically require significant amounts of time which can range up to 12-24 hours to complete. Depending on the particular process steps chosen a relatively complex series of reaction vessels may be required. Exposure of the polymer to high temperatures for long periods of time may also cause some degradation.
Therefore simpler, faster and less expensive polymerization processes for aromatic polyesters are desired.
The molecular weight of polyesters made using other types of chemical intermediates have been increased in an extruder, see for instance M. H. Mack, NPE'88, Vol. 2 Conference Papers, p. 131 et seq. (1988), and M. H. Mack et al., Annual Technical Conference Society of Plastics Engineers 1987, p. 136-139(1987). Besides the chemistry of the polymerization reactions being different, in these papers the polyesters are not made in the extruder from the monomers.